1. Field of the Invention
The present invention is related to structural rehabilitation and enhancement and, more particularly, to improving the structural capacity of structures by external reinforcements.
2. Description of Related Art
Construction materials, non-limiting examples of which may include wood, steel, concrete, etc. are routinely used in building structures. Most construction materials tend to deteriorate due to various reasons, including applied stresses, which weaken the built structure. Accordingly, it is sometimes necessary to reinforce an already constructed structure to improve or enhance its strength.
Some of the most common conventional methods for reinforcing already existing structural members are by mounting of reinforcements to the tension side thereof. Non-limiting examples of reinforcements may include the addition of steel plates or other supporting members such as additional columns or posts, beams, girders, or even additional walls, composite reinforcement materials, or any combination thereof to the tension side of the structural members for reinforcement.
As illustrated in prior art FIGS. 1A to 1D, under load conditions 106, a structure 102 must withstand horizontal forces 108 and 110, which are generally parallel along the neutral axis 112 of the structure 102. The horizontal forces may comprise of tensile forces 108 that tend to pull the structure 102, which produce elongation, and compression forces 110 that tend to push or compress the structure 102. Under heavy load conditions, the structure 102 must in addition withstand vertical forces 106 that are normal to the structure 102, which produce bending moments. Accordingly, as exemplarily illustrated in FIG. 1C, the exemplary tension zone for the exemplary structure 102 is at its underside 114, and the compression zone is located at its upper side 116. As more generally illustrated in FIG. 1D, the tension zones and the compression zones need not be limited to the underside 114 or the upper side 116 of a structure 102, but are found at any location where tensile forces 108 and the compression forces 110 exist, including, for example, tension zones 120 at the top of the supporting columns (negative load) 104.
In general, any element with a higher tensile strength (and relatively low compression strength) placed on top of a load-bearing element with lower tensile strength (and relatively high compression strength) will not provide adequate strength for the load-bearing element underneath (except in negative load zone). That is, for example, a deteriorating concrete slab (a load bearing element with low tensile strength, high compression strength) placed underneath a fiber reinforced polymer composite (with high tensile strength and low compression strength) will continue to deteriorate, crumble, and eventually buckle (in positive load zone). Therefore, it is most common to place reinforcing material with high tensile strength on the tension side of structures rather than on the compression side.
As an example, and as described and illustrated in the prior art U.S. Pat. No. 5,711,834 to Saito, the entire disclosure of which is expressly incorporated by reference in its entirety herein, the most common method for reinforcing structural members such as concrete slabs is the mounting of reinforcements to the underside of a slab, such as the underside of a bridge. However, as further described in Saito, this method was found to be unsuitable or impractical for reinforcement of heavily traveled topside surfaces such as the road surface of the bridge. To reinforce the topside, Saito places a unidirectional reinforcing fiber (high tensile strength) on top of thermosetting resin, which, in turn, is placed on top surface of a concrete slab (high compression strength). With this arrangement, the load-bearing element is the concrete slab with the unidirectional reinforcing fiber sheet placed on top of the concrete slab. The unidirectional reinforcing fiber is a low compression strength material and, therefore, cannot protect the underlying concrete, which has low tensile strength. The asphalt used on top of the fiber sheet is to protect the fiber form wear and tear and the outside environment, including heat, moisture, etc. Due to the nature of asphalt being a viscoelastic material, it is well known in the art that asphalt does not add much to the compression strength of structures because of its negligible and inconsistent compression and tensile strengths.
As a further example for the use of composites in reinforcing the tension-side of a structure, the U.S. Pat. No. 6,806,212 to Fyfe discloses that structures that must bear great weight, such as pillars, walls, or bridge spans, are often constructed from concrete and concrete is very strong under compression, so can support its own weight as well as the weight of other structural elements, people, vehicles, and equipment. Fyfe, the entire disclosure of which is expressly incorporated by reference in its entirety herein, further discloses that concrete is not strong under tension, though, and is a brittle material. Iron reinforcing rods are often embedded in concrete to increase the overall tensile strength. Nonetheless, to provide reinforcement, Fyfe discloses a coating method for strengthening the tension side of a concrete wall using composite material (tension side being the exploding side with the positive bending moment generated due to the direction of the exploding force).
As still another example for the use of composites in reinforcing the tension side of a structure, the U.S. Patent Application Publication 2006/0230985 to Derrigan et al. discloses that composite reinforcement materials have been used to strengthen existing concrete and masonry structures on the tension side. As further disclosed in Derrigan et al., while there are a number of advantages to using composite reinforcement materials, the composite reinforcement materials are generally much more combustible than concrete or masonry and, under the conditions of a fire, decrease the overall strength of the structure. To overcome this deficiency, Derrigan et al. insulates the fiber reinforced composite materials by a hydraulic cementitious material applied over the fiber reinforced polymer composite material, which does not add compression or tensile strength, but only provides added strength in terms of insulation for protection against heat or fire, comparable to gypsum or intumescent insulating materials, which have low compression and tensile strengths.
As is obvious from the prior art, conventional methods of reinforcements such as those taught by Saito, Fyfe, and Derrigan et al. base the determination factor for the location of the application of reinforcements onto a structure on the criteria of “heavy use” or tension zones, which is not practical or applicable to all structures. As is well known in the art for example, one of the tension zones of a roof structure is between two posts or columns that hold up the roof, with the tension zone being the underside of the roof (the ceiling) between the two posts or columns (similar to area 114 illustrated in FIG. 1C). However, as is common, a ceiling of a structure may include decorative features or other infrastructures such as piping, electrical, or other utilities, making the reinforcement on the tension side of the roof (the ceiling) impossible or at best, very costly. Further, in general, it is always more difficult in terms of access and cost to work against gravity (e.g., reinforcing a ceiling, which is the tension side of roof structure).
Accordingly, in light of the current state of the art and the drawbacks to current reinforcement methodologies mentioned above, a need exists for reinforcement of structures that would allow for strong reinforcement on the tension and or the compression sides of a load bearing structure so to avoid interference with any possible existing infrastructure (e.g., utilities) and, which would also provide both high compression and tensile strength, but without the addition of much bulk or weight.